Cooling system for a fuel cell stack

ABSTRACT

A fuel cell cooling system is provided with a coolant pump for pumping fluid through coolant flow field passages in a fuel cell stack. A pressure control mechanism is provided for maintaining a pressure level within the fuel cell stack for causing a phase change of the coolant within the stack. Allowing the coolant to change phase to a gas inside the stack reduces the amount of coolant needed to cool the fuel cell stack and thereby reduces the energy needed to pump the coolant through the fuel cell stack.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/152,858 filed on May 22, 2002, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to fuel cells and more particularly toa cooling system for a fuel cell stack that allows a liquid coolant tochange phase inside the fuel cell stack in order to reduce the pumpingpower required to circulate the coolant and provide more uniformtemperatures within each cell of a fuel cell stack.

BACKGROUND OF THE INVENTION

Fuel cells have been used as a power source in many applications. Forexample, fuel cells have been proposed for use in electrical vehicularpower plants to replace internal combustion engines. In proton exchangemembrane (PEM) type fuel cells, hydrogen is supplied to the anode of thefuel cell and oxygen is supplied as the oxidant to the cathode. Theoxygen can be either a pure form (O₂) or air (a mixture of O₂ and N2).PEM fuel cells include a membrane electrode assembly (MEA) comprising athin, proton transmissive, non-electrically conductive, gas impermeable,solid polymer electrolyte membrane having the anode catalyst on one faceand the cathode catalyst on the opposite face. The MEA is sandwichedbetween a pair of non-porous, electrically conductive elements or plateswhich (1) serve as current collectors for the anode and cathode, and (2)contain appropriate channels and/or openings formed therein fordistributing the fuel cell's gaseous reactants over the surfaces of therespective anode and cathode catalysts.

The term “fuel cell” is typically used to refer to either a single cellor a plurality of cells (stack) depending on the context. A plurality ofindividual cells are typically bundled together to form a fuel cellstack and are commonly arranged in electrical series. Each cell withinthe stack includes the membrane electrode assembly (MEA) describedearlier, and each such MEA provides its increment of voltage. By way ofexample, some typical arrangements for multiple cells in a stack areshown and described in U.S. Pat. No. 5,663,113.

The electrically conductive plates sandwiching the MEAs may contain anarray of grooves in the faces thereof that define a reactant flow fieldfor distributing the fuel cell's gaseous reactants (i.e., hydrogen andoxygen in the form of air) over the surfaces of the respective cathodeand anode. These reactant flow fields generally include a plurality oflands that define a plurality of flow channels therebetween throughwhich the gaseous reactants flow from a supply header at one end of theflow channels to an exhaust header at the opposite end of the flowchannels.

In a fuel cell stack, a plurality of cells are stacked together inelectrical series while being separated by a gas impermeable,electrically conductive bipolar plate. In some instances, the bipolarplate is an assembly formed by securing a pair of thin metal sheetshaving reactant flow fields formed on their external face surfaces.Typically, an internal coolant flow field is provided between the metalplates of the bipolar plate assembly. Various examples of a bipolarplate assembly of the type used in PEM fuel cells are shown anddescribed in commonly-owned U.S. Pat. No. 5,766,624.

Fuel cell stacks produce electrical energy efficiently and reliably.However, as they produce electrical energy, losses in theelectrochemical reactions and electrical resistance in the componentsthat make up the stack produce waste thermal energy (heat) that must beremoved for the stack to maintain a constant optimal temperature.Typically, the cooling system associated with a fuel cell stack includesa circulation pump for circulating a single-phase liquid coolant throughthe fuel cell stack to a heat exchanger where the waste thermal energy(i.e., heat) is transferred to the environment. The two most commoncoolants used are de-ionized water and a mixture of ethylene glycol andde-ionized water. The thermal properties of these typical liquidcoolants require that a relatively large volume be circulated throughthe system to reject sufficient waste heat in order to maintain aconstant stack operating temperature, particularly under maximum powerconditions. Large amounts of electrical energy are required to circulatethe coolant, which reduces the overall efficiency of the fuel cell powersystem. To this end, it is desirable to reduce the amount of coolantneeded to cool a fuel cell stack and thereby reduce the amount ofpumping power required.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a cooling system forchanneling coolant through a fuel cell. The cooling system includes aplate having a first side defining a reactant flow field and a secondside defining a coolant flow field. The coolant flow field has inlet andoutlet passages with a source of liquid coolant connected to the inletpassage. A pressure control mechanism is provided for maintaining apressure at the outlet passage of the coolant flow field at a pressurethat causes the liquid coolant to boil within the coolant flow field.Allowing the coolant to change phase to a gas inside the stack reducesthe amount of coolant needed to cool the fuel cell stack. The energyneeded to change a liquid to a gas is much greater than the heatcarrying capacity of the liquid. As a result, the amount of coolantneeded to cool a stack, and thereby the amount of coolant that must bepumped through the system is reduced as is the parasitic load on thesystem.

The present invention enables improved temperature uniformity andincreased radiator/condenser efficiency.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is an exploded isometric view of a PEM fuel stack;

FIG. 2 is a schematic view of the cooling system according to theprinciples of the present invention for use with the PEM fuel cell stackshown in FIG. 1;

FIG. 3 graphically illustrates the boiling curve relationship betweentemperature and pressure for a water/methanol coolant mixture; and

FIG. 4 is a perspective view of the coolant channels within the stack.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

Before further describing the invention, it is useful to understand anexemplary fuel cell system within which the invention operates.Specifically, FIG. 1 schematically depicts a PEM fuel cell stack havinga pair of membrane-electrode assemblies (MEAs) 4 and 6 separated fromeach other by a non-porous, electrically-conductive, liquid-cooledbipolar plate assembly 8. Each MEA 4 and 6 has a corresponding cathodeface 4 a, 6 a and an anode face 4 b and 6 b. MEAs 4 and 6 and bipolarplate assembly 8 are stacked together between non-porous,electrically-conductive, liquid-cooled monopolar end plate assembly 14and 16. Steel clamping plates 10 and 12 are provided for enclosing theexemplary fuel cell stack. Connectors (not shown) are attached toclamping plates 10 and 12 to provide positive and negative terminals forthe fuel cell stack. Bipolar plate assembly 8 and end plate assemblies14 and 16 include corresponding flow fields 20, 22, 18 and 24, eachhaving a plurality of flow channels formed in the faces thereof fordistributing fuel and oxidant gases (i.e., H₂ and O₂) to the reactivefaces of MEAs 4 and 6. Nonconductive gaskets or seals 26, 28, 30, and 32provide a seal and electrical insulation between the several plates ofthe fuel cell stack.

With continued reference to FIG. 1, porous, gas permeable, electricallyconductive sheets 34, 36, 38 and 40 are shown to be pressed up againstthe electrode faces of MEAs 4 and 6 and serve as primary currentcollectors for the electrodes. Primary current collectors 34, 36, 38 and40 also provide mechanical supports for MEAs 4 and 6, especially atlocations where the MEAs are otherwise unsupported in the flow fields.

End plates 14 and 16 press up against primary current collector 34 oncathode face 4 a of MEA 4 and primary current collector 40 on anode face6 b of MEA 6 while bipolar plate assembly 8 presses up against primarycurrent collector 36 on anode face 4 b of MEA 4 and against primarycurrent collector 38 on cathode face 6 a of MEA 6. An oxidant gas, suchas oxygen or air, is supplied to the cathode side of the fuel cell stackfrom a storage tank 46 via appropriate supply plumbing 42. Similarly, afuel, such as hydrogen, is supplied to the anode side of the fuel cellfrom a storage tank 48 via appropriate supply plumbing 44. In apreferred embodiment, oxygen tank 46 may be eliminated, such thatambient air is supplied to the cathode side from the environment.Likewise, hydrogen tank 48 may be eliminated and hydrogen supplied tothe anode side from a reformer which catalytically generates hydrogenfrom methanol or a liquid hydrocarbon (e.g., gasoline). While not shown,exhaust plumbing for both the H₂ and O₂/air sides of MEAs 4 and 6 isalso provided for removing H₂—depleted anode gas from the anode reactantflow field and O₂—depleted cathode gas from the cathode reactant flowfield.

Coolant supply plumbing 50, 52, and 54 is provided for supplying aliquid coolant from an inlet header (not shown) of the fuel cell stackto the coolant flow fields of bipolar plate assembly 8 and end plates 14and 16. The coolant flow fields of the bipolar plate assembly 8 and endplates 14 and 16 include long narrow channels 56 defining coolantpassages within the plates 8, 14, and 16. As shown in FIG. 1, coolantexhaust plumbing 58, 60, and 62 is provided for exhausting the heatedcoolant discharged from bipolar plate assembly 8 and end plates 14 and16 of the fuel cell stack.

FIG. 2 is a schematic diagram of a phase-change cooling system accordingto the principles of the present invention. As shown in FIG. 2, a fuelcell stack 70, such as the one shown in FIG. 1, is provided. A coolingsystem 72 includes a pump 74 which provides liquid coolant to the fuelcell stack 70 through coolant passage 76. A pressure control valve 78 isprovided at the exhaust end of the fuel cell stack 70 and aradiator/condenser 80 is provided downstream of the pressure controlvalve 78 for cooling the coolant (in liquid and vapor mixture form) andcondensing it back to a liquid form for return to the pump 74. Anaccumulator 86 is provided upstream of the pump 74 to remove bubblesfrom the coolant fluid before the pump 74. The accumulator 86 mayoptionally be provided with a dehydrator functionality which wouldremove water if the chosen fluid reacts adversely to water (i.e., theneeded properties change when mixed with water). A controller 82 isprovided for controlling the pressure control valve 78 in response to atemperature of the stack 70 as determined by a temperature sensor 84.The pressure is controlled such that for the measured temperature level,a pressure is maintained that causes the coolant to boil within thestack. The control 82 may include a processor (CPU) or dedicatedcircuitry for carrying out this function.

Liquid coolant exits the coolant pump 74 at an elevated pressure andenters the fuel cell stack 70. While in the stack, a fraction of thecoolant boils at a temperature determined by the pressure of thecoolant. As best illustrated in FIG. 4, the coolant channels 90 withinthe stack are designed to accommodate the expansion of some of theliquid to a gaseous state minimizing the increased pressure drop broughton by the increase in volumetric flow of two phase fluid. The coolantchannels 90 include a narrow liquid inlet 92 and a series of alternatingdirection serpentine channel segments 94 a-e that progressively widenfrom the inlet 92 to the outlet 96. The widening channel design ensuressomewhat uniform coolant distribution which in turn avoids regions wherethe coolant boils away completely causing hot spots. The coolant emergesfrom the stack as a two-phase mixture of liquid and vapor coolant. Themixture enters the pressure control valve 78 that is used to control thesystem pressure. Next, the coolant enters the radiator/condenser 80where the coolant changes back to a liquid. Upon exiting theradiator/condenser, the coolant returns to the coolant pump 74.

The present invention reduces the amount of coolant needed to cool afuel cell stack by allowing a liquid coolant to change phase to a gasinside the stack. The energy needed to change a liquid to a gas is muchgreater than the heat carrying capacity of the liquid. As a result, theamount of coolant needed to cool a stack, and thereby the amount ofcoolant that must be pumped through the system, is reduced. Thus, theparasitic load on the system that is typically required to pump thelarge amounts of coolant is also reduced. Test results have shown that amixture of 40 percent of methanol in water which was allowed to changephase in a fuel cell stack resulted in a pumping power reduction from1,000 watts to 200 watts in an 85 kilowatt fuel cell power system. Inother words, one-fifth of the pumping power was required when thepressure of the coolant was regulated such that some of the coolant wasallowed to change phase within the fuel cell stack according to theprinciples of the present invention.

Using a cooling system where the coolant boils has benefits in additionto reduced system parasitic losses. These benefits include improvedtemperature control reduced, improved temperature uniformity, andincreased radiator/condenser efficiency. When a liquid changes to a gas;i.e., boils, it does so at a single temperature. The local pressure ofthe liquid determines the temperature at which a liquid boils.Therefore, controlling the pressure control valve to maintain apredetermined pressure drop in a stack, specifies the temperaturegradient, and changing the pressure of the cooling loop changes thestack temperature. Increasing the pressure will increase the stacktemperature while lowering the pressure reduces the stack temperature,all the while the temperature gradient across the stack remains thesame. The relationship between pressure and boiling temperature for awater-methanol mixture is shown in FIG. 3. It should be understood thatother liquids with similar boiling characteristics could be used.

Using a coolant that changes phase has the added benefit that itincreases the efficiency of the radiator/condenser used to reject thewaste thermal-energy to the environment. The increase in efficiency isdue to the constant temperature relationship of condensing fluids. Theconstant temperature in the radiator/condenser means the temperaturedifference between the coolant and the air used to remove the heat ismaintained instead of reduced, as it is in single phase heat transfer.The temperature difference between the coolant and the air is one of themajor factors determining radiator/condenser efficiency.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A cooling system, comprising: a heat exchange member defining acoolant passage therein, said coolant passage having an inlet end and anoutlet end; a source of liquid coolant delivered to said inlet end ofsaid coolant passage; and a pressure control mechanism for controlling apressure of said outlet end of said coolant passage at a predeterminedpressure level to cause coolant in said coolant passage to boil at anoperating temperature of the cooling system.
 2. The cooling system ofclaim 1, wherein said coolant passage includes a plurality of elongatedpassages.
 3. The cooling system of claim 1, further comprising a heatexchanger for cooling coolant received from said pressure controlmechanism.
 4. The cooling system of claim 1, wherein said pressurecontrol mechanism includes a pressure control valve.
 5. The coolingsystem of claim 1, wherein said source of liquid coolant includes apump.
 6. The cooling system of claim 1, wherein said coolant passageprogressively widens from an inlet end to an outlet end to accommodateliquid-to-gas expansion.